The present invention relates to a method for manufacturing a primary preform for optical fibres using an internal vapour deposition process, comprising the steps of:
i) providing a hollow glass substrate tube having a supply side and a discharge side,
ii) surrounding at least part of the hollow glass substrate tube by a furnace,
iii) supplying glass-forming gases to the interior of the hollow glass substrate tube via the supply side thereof,
iv) creating a reaction zone with conditions such that deposition of glass will take place on the inner surface of the hollow glass substrate tube, and
v) moving the reaction zone back and forth along the length of the hollow glass substrate tube between a reversal point located near the supply side and a reversal point located near the discharge side of the hollow glass substrate tube so as to form one or more preform layers on the inner surface of the hollow glass substrate tube, both of which reversal points are surrounded by the furnace.
A method as described in the introduction is known per se from U.S. Pat. No. 4,741,747. More in particular, the aforesaid patent discloses a method of fabricating optical preforms according to the PCVD method, wherein glass layers are deposited by moving a plasma back and forth between two points of reversal inside a glass tube whilst adding a reactive gas mixture to the tube at a temperature between 1100° C. and 1300° C. and a pressure between 1 and 30 hPa. The regions of nonconstant deposition geometry at the ends of the optical preform are reduced by moving the plasma nonlinearly with time in the area of at least one reversal point.
U.S. patent application US 2003/0017262 relates to an apparatus and method for manufacturing an optical fiber preform. From said US application it is known that two separate heat sources are positioned a predetermined distance apart, seen in the longitudinal direction of the substrate tube. The two heat sources are moved along the length of the substrate tube whilst maintaining the predetermined spacing during the MCVD (Modified Chemical Vapour Deposition) process.
U.S. Pat. No. 4,608,070 discloses a process for manufacturing a preform wherein the deposition process is carried out using a temperature profile, which temperature profile increases along the length of the substrate tube.
German Offenlegungsschrift DE 32 06 17 discloses a method for manufacturing a preform wherein a graphite furnace surrounds a substrate tube, which graphite furnace is provided with an additional heat source, which heat source functions as a pre-heater for the gas mixture to be supplied to the substrate tube. The two heat sources can be moved over the tube along the length thereof while maintaining the spacing between the two heat sources.
German Offenlegungsschrift DE 36 19 379 relates to a method and device for manufacturing a preform, wherein two co-axially arranged tubes can be heated and cooled independently so as to thus effect a temperature change.
U.S. Pat. No. 4,331,462 relates to a method for manufacturing a preform by means of an MCVD process, using a so-called tandem heating zone made up of a zone I and a zone II.
An optical fibre consists of a core and an outer layer surrounding said core, also referred to as cladding. The core usually has a higher refractive index than the cladding, so that light can be transported through the optical fibre.
The core of an optical fibre may consist of one or more concentric layers, each having a specific thickness and a specific refractive index or a specific refractive index gradient in radial direction.
An optical fibre having a core consisting of one or more concentric layers having a constant refractive index in radial direction is also referred to as a step-index optical fibre. The difference between the refractive index of a concentric layer and the refractive index of the cladding can be expressed in a so-called delta value, indicated Δi % and can be calculated according to the formula below:
            Δ      i        ⁢                  ⁢    %    =                    n        i        2            -              n        cl        2                    2      ⁢              n        i        2            
where:
ni=refractive index value of layer i
ncl=refractive index value of the cladding
An optical fibre can also be manufactured in such a manner that a core having a so-called gradient index refractive index profile is obtained. Such a radial refractive index profile is defined both with a delta value Δ% and with a so-called alpha value α. To determine the Δ% value, use is made of the maximum refractive index in the core. The alpha value can be determined by means of the formula below:
      n    ⁡          (      r      )        =                    n        1            ⁡              (                  1          -                      2            ⁢            Δ            ⁢            %            ⁢                                          (                                  r                  a                                )                            α                                      )                    1      2      
where:
n1=refractive index value in the centre of het fibre
a=radius of the gradient index core [μm]
α=alpha value
r=radial position in the fibre [μm]
A radial refractive index profile of an optical fibre is to be regarded as a representation of the refractive index as a function of the radial position in an optical fibre. Likewise it is possible to graphically represent the refractive index difference with the cladding as a function of the radial position in the optical fibre, which can also be regarded as a radial refractive index profile.
The form of the radial refractive index profile, and in particular the thicknesses of the concentric layers and the refractive index or the refractive index gradient in the radial direction of the core determine the optical properties of the optical fibre.
A primary preform comprises one or more preform layers which form the basis for the one or more concentric layers of the core and/or part of the cladding of the optical fibre that can be obtained from a final preform. A preform layer is built up of a number of glass layers.
A final preform as referred to herein is a preform from which an optical fibre is made by using a fibre drawing process.
To obtain a final preform, a primary preform is externally provided with an additional layer of glass, which additional layer of glass comprises the cladding or part of the cladding. Said additional layer of glass can be directly applied to the primary preform. It is also possible to place the primary preform in an already formed glass tube, also referred to as “jacket”. Said jacket may be contracted onto the primary preform. Finally, a primary preform may comprise both the core and the cladding of an optical fibre, so that there is no need to apply an additional layer of glass. A primary preform is in that case identical to a final preform. A radial refractive index profile can be measured on a primary preform and/or on a final preform.
The length and diameter of a final preform determine the maximum length of optical fibre that can be obtained from the final preform.
To decrease the production costs of optical fibres and/or increase the output per primary preform, the object is to produce, on the basis of a final preform, a maximum length of optical fibre that meets the required quality standards.
The diameter of a final preform can be increased by applying a thicker layer of additional glass to a primary preform. Since the optical properties of an optical fibre are determined by the radial refractive index profile, the thickness of the layer of additional glass must at all times be in the correct proportion to the layer thickness of the preform layers of the primary preform that will form the core, more in particular the one or more concentric layers of the core in the optical fibre. Consequently, the layer thickness of the glass layer additionally applied to the primary preform is limited by the thickness of the preform layers being formed by means of the internal vapour deposition process.
The length of a final preform can be increased by increasing the length, more in particular the usable length, of a primary preform. The “usable length” is to be understood to be the length of the primary preform along which the optical properties remain within a predetermined tolerance limits, which tolerance limits have been selected so that optical fibres that meet the desired quality standards are obtained.
To determine the usable length of the primary preform, a radial refractive index profile is measured at a number of positions along the length thereof, after which it is possible, based on said measurements, to draw up a so-called longitudinal refractive index profile and a longitudinal geometry profile for each preform layer.
Thus a “longitudinal refractive index profile” is to be understood to be a graphic representation of the refractive index of a preform layer as a function of the longitudinal position in the primary preform.
A “longitudinal geometry profile” is to be understood to be a graphic representation of the thickness of a preform layer as a function of the longitudinal position in the primary preform.
One of the factors that adversely affect the usable length of a primary preform it is so-called “taper”. The term “taper” is to be understood to be a deviation of the optical and/or geometric properties of the primary preform in regions near the ends thereof. A distinction is made between optical taper and geometric taper.
Optical taper relates to refractive index deviations, whilst geometric taper relates to deviations of the layer thickness of the preform layer.
If a primary preform is built up of several preform layers, the optical and geometric taper of the preform layers may be different from each other.